1. Field of the Invention
The present invention relates to a control apparatus for an on-vehicle generator mounted on vehicles, such as automobiles and autotrucks, and an on-vehicle power supply system using the control apparatus.
2. Description of the Related Art
Recently, there has been a rise in demands for both high power and high efficiency of on-vehicle generators. To meet such demands, various countermeasures have been taken. One proposed countermeasure is to use an on-vehicle generator in which a Rundel-type pole core functioning as a rotor is provided and a permanent magnet is placed in a clearance between uniform portions of the core so as to increase effective magnetic flux.
However, there is a fear that such permanent magnet may be broken due to various reasons such as vibration transmitted from the car body. In addition, both of the number of parts and the number of man-hours are obliged to rise. Thus the conventional on-vehicle generator has encountered the problems resulting from durability and production cost. It has therefore been considered that, in usual cases, improvement of a field winding of the generator is more advantageous than employment of the permanent magnet.
In order to improve the field winding for high output power and high efficiency, there is known a design technique of strengthening an excitation ampere turn value of the field winding so that a maximized excitation field is obtained. Under such a circumstance, a growing demand for shortening a time constant of the field winding has been created as another design technique. Thus a current main stream in the design begins with selecting, as the field winding, smaller-number-of-turns and low-resistance winding, rather than a multitudes-of-turns and high-resistance winding.
In addition to the foregoing demands for high-power and high-efficiency, a demand for a compact generator is still persistent, so that a trend in the design is making the magnetic poles of the generator compact in size and less in weight. When such a design is made, a magnetic path for the field is forced to be smaller in the cross section, thus a magnetic permeance value being smaller, thus the magnetic path being apt to be saturated easily. Hence, to generate a necessary amount of interlinkage magnetic flux against a large amount of magnetic resistance, the excitation current tends to be increased more.
By the way, in the general on-vehicle generator, the excitation current is produced by using an output current from an in-vehicle battery or by using part of an output current generated by an on-vehicle generator itself. Hence the larger the excitation current, the higher a loss due to the excitation, thus facing another problem.
FIG. 1 explains connections of a field winding equipped in a conventional on-vehicle generator. As shown in FIG. 1, the conventional on-vehicle generator has a switch 102 made up of a power transistor connected in series to a field winding 100 and a free wheel diode 104 connected in parallel to the field winding 100. Controlling the switch 102 to be turned on/off suppresses an excitation current flowing through the field winding 100, providing an output voltage limited within a predetermined range of allowance.
FIG. 2 shows temporal changes in both the excitation current and a battery current in the conventional on-vehicle generator including the circuitry shown in FIG. 1. As understood from FIG. 2, a current (battery current) flows from the battery 106 to the field winding 100 during an interval of time when the switch 102 is in the on-state. Energy supplied by this battery current is partly preserved as magnetic energy by the field winding 100. During an interval of time when the switch 102 is in the off-state, this magnetic energy causes the excitation current to circulate through a closed loop formed by the field winding 100 and free wheel diode 104. While circulating along the closed loop, the excitation current is converted into Joule heat due to both of the resistance of the field winding 100 and a forward voltage drop across the free wheel diode 104, so that magnetic energy is dissipated.
Therefore, when it is desired to raise the amount of the excitation current to realize a compact, high-output-power, and high-efficiency on-vehicle generator as described above, there are still other problems that the capacity of the power supply should be increased and a generation loss is increased because there is an increase in the dissipation energy converted into heat after the temporal storage in the field winding.